Natural gas and refinery gas streams are commonly contaminated with sulfur-containing compounds such as hydrogen sulfide (H2S) and/or carbonyl sulfide (COS) and carbon dioxide (CO2). If substantial amounts of H2S are present, regulatory restrictions dictate special precautions must be taken to purify the gas streams. The first step of the H2S removal process from the H2S-containing streams is accomplished by an acid-gas removal unit which removes substantial amounts of H2S and CO2 from the acidic-gas containing streams. The off-gas from the acid-gas removal unit is mainly H2S and CO2. The sulfur from this off-gas stream is usually removed by the Claus reaction which produces salable elemental sulfur. After a ‘tail-gas’ treatment to further reduce the sulphur content, the remaining CO2 may be safely vented to the atmosphere. However, there has been increasing concern about the damage caused by CO2 and this has led to an increased demand to reduce the emission of CO2 to the atmosphere.
Typically, separation of CO2 and H2S from streams containing acidic gas is achieved by the chemical absorption process employing liquid amine solutions, such as monoethanolamine (MEA), diethanolamine (DEA) or methyldiethanolamine (MDEA). In this process the CO2 reacts with the liquid amine solution to form a carbamate, while H2S reacts with the amine solution to form (amine)H+ and bisulfide (SH−) species. Upon heating, the carbamate and (amine)H+ species decompose to release the absorbed CO2 and H2S and produce a regenerated amine solution. Disadvantageously with this process, however, sulfur-containing compounds such as SO2, COS and/or CS2, if present in the feed stream, react with the liquid amine absorbent and a higher temperature is required to regenerate the amine solution. SO2 also reacts with the amine to form sulphates which necessitates partial replacement of the amine.
Liquid alkoxylated amines, such as diisopropanolamine, have been used for CO2 removal from streams containing acidic gases. U.S. Pat. No. 4,044,100 described the use of liquid mixtures of diisopropanolamine and polyethylene glycol for acid gas removal from gaseous streams.
There are many fields of applications in which it is required to remove H2S and CO2 from streams containing acidic gases. U.S. Pat. No. 4,553,984 describes a process for the removal of CO2 and H2S, simultaneously, from streams containing acidic gases wherein the stream is brought into counter flow contact with an aqueous of methyldiethanolamine (MDEA) at a pressure of 10-110 bars. Nevertheless, there are different applications in which it is required to reduce the H2S to a very low level without essential removal of CO2; therefore, solvents with high H2S-absorbing power are desired. U.S. Pat. No. 5,277,884 disclosed a process for selective removal of H2S from streams containing both H2S and CO2 acidic gases. The process according to that invention comprises contacting the acidic gas containing stream with a solvent that comprises a mixture of N-methylpyrrolidone (NMP) and dodecane.
The acid gas removal process utilizing liquid amine solutions is costly and energy-intensive because the liquid amine solution has a limited life time due to its degradation through oxidation. Furthermore, the high corrosivity of the utilized amine makes it prohibitive to use high concentrations of the amine solutions. Therefore, new acidic gas capture technology utilizing thermally stable solid sorbents has increasingly received attention due to its potential for reducing corrosion and energy cost and improving mass/heat transfer efficiency. Such technology is based on the ability of a porous solid sorbent to reversibly adsorb the CO2 and H2S from the acidic gas containing streams at high pressure.
U.S. patent application Ser. No. 13/399,911 filed Feb. 17, 2012 relates to a process for a acidic gas recovery from acidic gas containing streams employing a class of novel thermally stable amine adducts (sorbents). The regenerable sorbents described in that process had high CO2 and H2S absorption capacity and comprised a porous solid support, a cross-linked amine and a polyol reactive toward the utilized amine. The sorbents according to this invention enable acidic gas absorption/desorption cycles at various temperatures and pressures. Advantageously, the absorption/desorption cycles could be conducted at a pressure of 1500 psig and a temperature of 130° C., so that the CO2 at this condition was ready for direct downhole storage or pipelining at greatly reduced compression costs. In addition the adsorption could take place at low pressure with desorption at high pressure.
Typically, the desorbed gas stream from an acid-gas removal unit is mainly H2S and CO2 and the sulfur is usually removed by the Claus process. In the first step in the Claus process, one third of the hydrogen sulfide present in the feed stream is oxidized to sulfur dioxide, SO2, by the reaction as follows:H2S+O2=SO2+H2 In the second step, the remaining H2S and the SO2 are reacted in the presence of a Claus catalyst to form elemental sulfur in a Claus reactor according to Reaction 1:2H2S+SO2=2H2O+3S Claus reaction  1.
The Claus reaction is limited by thermodynamic equilibrium and only a portion of the total sulfur can be produced. Therefore, multiple stages with sulfur condensation between the stages are needed in order to increase the sulfur recovery factor. The effluent gas from a series of reactors in a Claus plant contains varying amounts of different compounds including sulfur vapor, sulfur dioxide, un-reacted H2S, carbonyl sulfide (COS), and/or carbon disulfide (CS2). Carbon disulphide is formed according to Reaction 2:CH4+4S→CS2+2H2S High temperature Claus furnace or combustion reaction  2.
Removal of the sulfur content of the off-gas streams from the Claus process is accomplished by catalytic reduction with hydrogen to convert the sulfur compounds to H2S, absorption of the H2S produced with an additional amine system and then recycling the desorbed gas to the Claus plant. This process is operable as long as the concentration of the CO2 is up to 15% and H2S is above 50% by volume in the feed stream. However, if the H2S/CO2 feed gas stream to Claus process contains less than 40% by volume H2S, the Claus plant becomes difficult to operate with respect to the thermal zone and special considerations have to be taken when combusting part of H2S to SO2 as required for the Claus reaction. These operational difficulties mainly arise from the fact that the required temperatures for the combustion of H2S cannot be reached in the thermal zone. Therefore, the off-gas stream from the Claus plant is burned with air to convert all sulfur-containing compounds in the stream to SO2 before discharge into the atmosphere. As the environmental requirements are becoming stricter, the SO2 emission limit is being lowered, giving rise to the challenge of how to reduce or completely eliminate SO2 emissions. Consequently, another sulfur removal process is needed that can handle H2S/CO2 feed gas streams containing CO2 of concentrations greater than 15% and H2S of a concentration less than 40% by volume.
The direct oxidation of H2S to elemental sulfur using oxidation catalysts has gained broad acceptance for achieving high sulfur removal efficiency. U.S. Pat. No. 4,197,277 describes a process for the oxidation of H2S to elemental sulfur by the following H2S Oxidation Reactions 3 and 4:H2S+0.5O2→S+H2O H2S Partial oxidation  3.H2S+1.5O2→SO2+H2O H2S Complete oxidation  4.
According to U.S. Pat. No. 4,197,277, the H2S-containing gas is passed with an oxygen-containing gas over a catalyst which comprises iron oxide and vanadium oxide as active materials and aluminum oxide as a support material. The catalyst described in that Patent gives rise to at least a partial Claus equilibrium, so that SO2 formation cannot be prevented. Similarly, U.S. Pat. No. 5,352,422 describes a process for oxidizing the un-reacted H2S in the Claus tail gas to elemental sulfur. The patent describes a catalyst prepared by impregnation of an iron containing solution or an iron/chromium-containing solution into several carriers followed by calcinations in air at 500° C.
U.S. Pat. No. 4,818,740 disclosed a catalyst for the H2S oxidation to elemental sulfur, the use of which prevents the reverse Claus reaction to a large extent. The catalyst according to that patent comprises a support of which the surface exposed to the gaseous phase does not exhibit any alkaline properties under the reaction conditions, while a catalytically active material is applied to this surface. An improvement of the method disclosed in '740 is disclosed in European Patent 409,353. This patent relates to a catalyst for the selective oxidation of sulfur-containing compounds to elemental sulfur, comprising at least one catalytically active material and optionally a support. The described catalyst exhibits substantially no activity towards the reverse Claus reaction under the reaction conditions.
The H2S direct oxidation to elemental sulfur is suitable for gas streams comprising high concentrations of CO2 and low concentrations of H2S. Nevertheless, the total sulfur removal efficiency decreases if carbon monoxide or COS gases are present in the feed stream. Carbon monoxide, if present in the feed gas streams, undergoes side reactions during the H2S direct oxidation to form COS. In addition, CO2 may also react with H2S to form COS during direct oxidation reaction:CO+S→COS  5.CO+H2S→COS+H2  6.3CO+SO2→COS+2CO2  7.H2S+CO2→COS+H2O  8.
U.S. patent application Ser. No. 13/399,710 filed Feb. 17, 2012 entitled “Removal of Sulfur Compounds from a Gas Stream” relates to a process for simultaneously oxidizing H2S to elemental sulfur and hydrolyzing COS to H2S in the presence of an oxidation catalyst and a feed gas stream containing CO of a concentration greater than 1% by volume and CO2 of a concentration greater than 14% by volume of the total feed gas flow. In this process, an H2S-containing stream was mixed with a molecular oxygen containing gas and then passed over an oxidation catalyst at a temperature of 220° C., a gas hourly space velocity of 1000 hr−1 and a pressure of 100 psig. The concentration of the COS produced decreased from 1900 ppm, using a dry gas stream, to 316 ppm upon using a feed stream containing greater than 10% water. The oxygen in the feed gas stream was adjusted to achieve the highest conversion of H2S to elemental sulfur and to deliberately produce an off-gas stream containing H2S/SO2 ratio of 2:1 which is ready as a feed gas stream for other sulfur removal units such as Crystasulf™1. Therefore, the process was operated at a relatively low sulfur yield of 78.1% and a total H2S conversion of 90.4%. 1 Trademark of URS CORPORATION for sulfur removal units.
In summary, high sulfur removal efficiency can be achieved by utilizing a multi-stage Claus process and off-gas post treatment. Importantly, however, this process is limited by the concentration of the CO2 in the gas stream and necessity of employing an H2S enrichment unit. Therefore, other sulfur recovery processes, such as the H2S direct oxidation process, have gained worldwide attention. In fact, the H2S direct oxidation to elemental sulfur process has become the cornerstone of the high sulfur recovery upon coupling with Claus process. Disadvantageously, however, the H2S direct oxidation process is still limited due to the process conditions and feed gas composition. As mentioned, a considerable amount of COS is produced when operating the H2S direct oxidation process, in a once-through mode, with sulfur-containing gas streams comprising CO and CO2 at a temperature above the sulfur dew point and a high pressure. Consequently, a robust sulfur removal process that can overcome the aforementioned difficulties is still needed.